In a product, speaker drivers are typically combined with enclosures. For example a wide range of electronic devices, such as desktop computer, laptop computer, tablet, and smartphone, have built-in speaker drivers. A driver may as well be mounted onto its own dedicated cabinet box, creating a standalone speaker product. Examples for this are the typical Hi-Fi or stereo loudspeakers which are part of a home Hi-Fi system or professional sound reinforcement systems typically used in concert venues. Other examples of drivers which are mounted onto enclosures are headphones, car stereo systems, and TV screens.
Buzzing sounds caused by the enclosure, the driver itself or a combination of the two is a common issue that arises when the driver is operating at considerable levels. In this disclosure, a “buzz” is considered to be an acoustic disturbance that is produced in large part due to a resonating mechanical component. The resonance is excited by an audio signal. When the resonance frequency of a mechanical component matches with an audio frequency component in the audio signal, the mechanical component experiences its largest amplitude of vibration. If the amplitude is large enough to cause one vibrating mechanical component to touch another component, a buzzing sound is produced. Additionally, even without a component actually touching other components, a component can emit a buzz which is simply caused by high-energy excitation that produces sound.
In a simplified model, music can be viewed as a sequence of harmonic partial tones. A harmonic partial tone of an audio signal is a frequency component of the audio signal that is an integer multiple of the fundamental frequency. For example, if the fundamental frequency is f, the harmonic partial tones have frequencies 2f, 3f, 4f . . . etc. The partial tones change over time with respect to their count, level and frequency. The majority of the energy of a musical signal is stored in its partial tones. This also holds for speech and animal sounds. Concentrating the energy of a communication signal (such as speech, music or animal sounds) into a few partial tones rather than spreading it over a large number of partial tones or a wider frequency spectrum is an energy efficient method of delivering a message across a noisy environment. In this disclosure, the terms “harmonic partial tone” and “partial tone” are used interchangeably.
In the case where music, speech or the like is being played back over a driver which is mounted on an enclosure whose structural resonances match with partial tones and the sound playback level is beyond a certain threshold, buzz is created. A buzz can however be masked by other components of the audio signal in which case they are not audible to a human being and therefore of little concern. On the other hand, if not masked, buzzes are very annoying.
A traditional way of mitigating the buzz problem is to apply changes to the physical structure of the enclosure. There are limits imposed by unit cost, weight, and product design however, often rendering a complete physical modifications-based elimination of buzzes unfeasible. If buzzes cannot be eliminated by the physical design, an audio signal processing approach referred to as equalization is commonly used, to reduce the levels of the frequency ranges that cause buzz.